The invention relates to a sequential x-ray crystal spectrometer for analyzing x-ray radiation from a sample wherein a curved analyzer crystal and a position-sensitive detector is provided for measuring x-ray radiation intensity.
Sequential x-ray crystal spectrometers for conventional construction are described e.g. in "Analytical X-Ray Systems Catalog, Siemens MP 42 (1980)", incorporated herein by reference. Essentially the fluorescent radiation emanating from a large-surface sample is brought to diffraction on a planar analyzer crystal, and the radiation intensity detected on a large-surface detector is measured as a function of the position (angle .theta.) of the analyzer crystal. The resolution of such a system is determined by the acceptance angle (e.g. 0.1.degree.) of a Soller collimator between sample and analyzer crystal. The geometric efficiency of the measurement results as the product of sample surface and solid angle, that is as the product of a large surface and a very small solid angle. The detector is here given only the role of recording the diffracted x-ray quanta. Information concerning at which location of the detector surface the diffracted x-ray quanta strike is without significance herein.
Also known is a multi-channel spectrometer (Siemens Advertising Brochure MRS400, Order No. E6892/1007), incorporated herein by reference, in which the principle of the "Seemann-Bohlin focusing geometrie" is employed. In the case of such a multichannel spectrometer, for each element to be analyzed, a permanently set curved analyzer crystal is employed which, together with diaphragms and a respective detector, defines one channel each. Since no parts need be moved, the ray or beam path is here readily evacuatable so that elements can be detected up to a very low atomic number (carbon). Entry and exit slits precisely define the ray trace so that the background can be kept very low. The x-ray output can here be indicated proportionally to the product of the entry slit and beam divergence, i.e. a small surface and a large solid angle. In the case of a multi-channel spectrometer, the x-ray output should have approximately the same order of magnitude as in the case of conventional sequence x-ray spectrometers.
An elegant solution for the sequential detection of a wide spectral range is the focusing linear spectrometer, such as has been realized in the so-called "scanner" of the multichannel spectrum "Siemens MRS400" system, incorporated herein by reference. Measurement can here be carried out with a fixed detector voltage and a fixed discriminator window, as a consequence of which higher orders of diffraction as well as uncorrelated stray radiation can be eliminated.
With a position-sensitive resolution detector (PSD), it is possible to detect simultaneously several elements in a focusing arrangement. A x-ray spectrometer with a PSD and a focusing analyzer crystal thus functions in a restricted wavelength range like a multi-channel spectrometer. The diffraction 2.theta. can be read off linearly on the focusing circle. A position-sensitive proportional counter (PSPC), however, permits no oblique beam incidence, so that its window must always be aligned perpendicularly to the crystal center. This inevitably leads to a violation of the focusing conditions and to a distortion of the location scale along the counter wire, whereby the distortion is proportional to the distance of detector/crystal center. In the case of a x-ray spectrometer with a PSPC with a focusing analzyer crystal, favorable conditions result for a precise measurement only in a restricted angular range about the detector center. In addition, there is the fact that different wavelengths are detected from different locations on the sample.
E. Bruninx (Philips' Research Reports 32 (1977) 253265 and Spektrochimica Acta 31B, (1976) Page 221, incorporated herein by reference) showed experiments with different geometric arrangements in the case of such a x-ray spectrometer with a PSPC and a focusing crystal, whereby a stationary PSD could resolve without difficulties, adjacent elements in the wavelength range of about 0.15 nm (CuK.alpha.). However, in comparison with the results thus achieved, an energy-dispersive spectrometer shows an improved performance which, although exhibiting a poorer resolution, delivers instead a more favorable peak-background ratio and substantially higher yields or efficiency.